This invention relates to a process for refining of a crude sodium hexafluorosilicate containing gypsum and phosphates as impurities.
Sodium hexafluorosilicate (sodium silicofluoride) is obtained largely as a by-product of wet process phosphoric acid and is of use for preparation of various fluorine compounds. However, sodium hexafluorosilicate of this origin is a crude material which contains a large amount of calcium sulfate dihydrate or gypsum and, in addition, various phosphates including not only soluble ones but also insoluble and sparingly soluble ones typified by iron and aluminum phosphates. Accordingly sufficient purification is necessary to render this material really valuable as an industrial fluorine source material, but unfortunately purification of the crude fluorosilicate has encountered difficulty and required complicated procedures fundamentally because of an inevitable matter that the crude fluorosilicate is obtained in the form of very fine crystals in which are entrapped impurity phosphates.
Among various refining processes hitherto proposed for sodium hexafluorosilicate, treatment with a sodium chloride solution is known as a relatively simple and industrially favorable process, which is based on the fact that the solubility of gypsum in a sodium chloride solution is distinctly greater than that in plain water, while sodium hexafluorosilicate exhibits a remarkably smaller solubility in the same solution than in plain water. When, for example, a mixture of sodium hexafluorosilicate and gypsum is dispersed in a 25 g/l aqueous solution of sodium chloride at room temperature, the gypsum in the mixture exhibits a solubility more than twice the solubility in plain water but the fluorosilicate exhibits a solubility less than one-tenth of its solubility in plain water. It is possible to achieve the removal of gypsum from the crude fluorosilicate near to satisfaction by this refining process, accompanied with removal of considerable portions of soluble impurities such as NaH.sub.2 PO.sub.4, Na.sub.2 HPO.sub.4, NaHSO.sub.4 and Ca(H.sub.2 PO.sub.4).sub.2. However, the purity of sodium hexafluorosilicate refined by this process is still unsatisfactory in view of current requirements from the markets related to the preparation of high purity fluorine compounds such as synthetic cryolite and aluminum fluoride. Besides, sodium hexafluorosilicate is obtained as very fine crystals when refined by conventional processes, causing a serious inconvenience to draining and drying of the refined material.